Re: [PATCH v3 3/6] ovl: Support escaped overlay.* xattrs

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On Thu, Sep 7, 2023 at 11:44 AM Alexander Larsson <alexl@xxxxxxxxxx> wrote:
>
> There are cases where you want to use an overlayfs mount as a lowerdir
> for another overlayfs mount. For example, if the system rootfs is on
> overlayfs due to composefs, or to make it volatile (via tmps), then
> you cannot currently store a lowerdir on the rootfs. This means you
> can't e.g. store on the rootfs a prepared container image for use
> using overlayfs.
>
> To work around this, we introduce an escapment mechanism for overlayfs
> xattrs. Whenever the lower/upper dir has a xattr named
> `overlay.overlay.XYZ`, we list it as overlay.XYZ in listxattrs, and
> when the user calls getxattr or setxattr on `overlay.XYZ`, we apply to
> `overlay.overlay.XYZ` in the backing directories.

Please use ""

>
> This allows storing any kind of overlay xattrs in a overlayfs mount
> that can be used as a lowerdir in another mount. It is possible to
> stack this mechanism multiple times, such that
> overlay.overlay.overlay.XYZ will survive two levels of overlay mounts,
> however this is not all that useful in practice because of stack depth
> limitations of overlayfs mounts.
>
> Signed-off-by: Alexander Larsson <alexl@xxxxxxxxxx>

With comment in ovl_listxattr() below fixed you may add:

Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>

> ---
>  fs/overlayfs/overlayfs.h |  7 ++++
>  fs/overlayfs/xattrs.c    | 78 +++++++++++++++++++++++++++++++++++++---
>  2 files changed, 81 insertions(+), 4 deletions(-)
>
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index dff7232b7bf5..736d7f952a8e 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -32,6 +32,13 @@ enum ovl_path_type {
>  #define OVL_XATTR_USER_PREFIX XATTR_USER_PREFIX OVL_XATTR_NAMESPACE
>  #define OVL_XATTR_USER_PREFIX_LEN (sizeof(OVL_XATTR_USER_PREFIX) - 1)
>
> +#define OVL_XATTR_ESCAPE_PREFIX OVL_XATTR_NAMESPACE
> +#define OVL_XATTR_ESCAPE_PREFIX_LEN (sizeof(OVL_XATTR_ESCAPE_PREFIX) - 1)
> +#define OVL_XATTR_ESCAPE_TRUSTED_PREFIX OVL_XATTR_TRUSTED_PREFIX OVL_XATTR_ESCAPE_PREFIX
> +#define OVL_XATTR_ESCAPE_TRUSTED_PREFIX_LEN (sizeof(OVL_XATTR_ESCAPE_TRUSTED_PREFIX) - 1)
> +#define OVL_XATTR_ESCAPE_USER_PREFIX OVL_XATTR_USER_PREFIX OVL_XATTR_ESCAPE_PREFIX
> +#define OVL_XATTR_ESCAPE_USER_PREFIX_LEN (sizeof(OVL_XATTR_ESCAPE_USER_PREFIX) - 1)
> +
>  enum ovl_xattr {
>         OVL_XATTR_OPAQUE,
>         OVL_XATTR_REDIRECT,
> diff --git a/fs/overlayfs/xattrs.c b/fs/overlayfs/xattrs.c
> index b8ea96606ea8..27b31f812eb1 100644
> --- a/fs/overlayfs/xattrs.c
> +++ b/fs/overlayfs/xattrs.c
> @@ -4,6 +4,18 @@
>  #include <linux/xattr.h>
>  #include "overlayfs.h"
>
> +bool ovl_is_escaped_xattr(struct super_block *sb, const char *name)
> +{
> +       struct ovl_fs *ofs = sb->s_fs_info;
> +
> +       if (ofs->config.userxattr)
> +               return strncmp(name, OVL_XATTR_ESCAPE_USER_PREFIX,
> +                              OVL_XATTR_ESCAPE_USER_PREFIX_LEN) == 0;
> +       else
> +               return strncmp(name, OVL_XATTR_ESCAPE_TRUSTED_PREFIX,
> +                              OVL_XATTR_ESCAPE_TRUSTED_PREFIX_LEN - 1) == 0;
> +}
> +
>  bool ovl_is_private_xattr(struct super_block *sb, const char *name)
>  {
>         struct ovl_fs *ofs = OVL_FS(sb);
> @@ -82,8 +94,8 @@ static int ovl_xattr_get(struct dentry *dentry, struct inode *inode, const char
>
>  static bool ovl_can_list(struct super_block *sb, const char *s)
>  {
> -       /* Never list private (.overlay) */
> -       if (ovl_is_private_xattr(sb, s))
> +       /* Never list non-escaped private (.overlay) */
> +       if (ovl_is_private_xattr(sb, s) && !ovl_is_escaped_xattr(sb, s))
>                 return false;
>
>         /* List all non-trusted xattrs */
> @@ -97,10 +109,12 @@ static bool ovl_can_list(struct super_block *sb, const char *s)
>  ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
>  {
>         struct dentry *realdentry = ovl_dentry_real(dentry);
> +       struct ovl_fs *ofs = OVL_FS(dentry->d_sb);
>         ssize_t res;
>         size_t len;
>         char *s;
>         const struct cred *old_cred;
> +       size_t prefix_len;
>
>         old_cred = ovl_override_creds(dentry->d_sb);
>         res = vfs_listxattr(realdentry, list, size);
> @@ -108,6 +122,9 @@ ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
>         if (res <= 0 || size == 0)
>                 return res;
>
> +       prefix_len = ofs->config.userxattr ?
> +               OVL_XATTR_USER_PREFIX_LEN : OVL_XATTR_TRUSTED_PREFIX_LEN;
> +
>         /* filter out private xattrs */
>         for (s = list, len = res; len;) {
>                 size_t slen = strnlen(s, len) + 1;
> @@ -120,6 +137,12 @@ ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
>                 if (!ovl_can_list(dentry->d_sb, s)) {
>                         res -= slen;
>                         memmove(s, s + slen, len);
> +               } else if (ovl_is_escaped_xattr(dentry->d_sb, s)) {
> +                       memmove(s + prefix_len,
> +                               s + prefix_len + OVL_XATTR_ESCAPE_PREFIX_LEN,
> +                               slen - (prefix_len + OVL_XATTR_ESCAPE_PREFIX_LEN) + len);
> +                       res -= OVL_XATTR_ESCAPE_PREFIX_LEN;
> +                       s += slen - OVL_XATTR_ESCAPE_PREFIX_LEN;

It's a bit hard to follow. how about:

res -= OVL_XATTR_ESCAPE_PREFIX_LEN;
name_len = slen - prefix_len - OVL_XATTR_ESCAPE_PREFIX_LEN;
s += prefix_len;
memmove(s, s + OVL_XATTR_ESCAPE_PREFIX_LEN, name_len + len);
s += OVL_XATTR_ESCAPE_PREFIX_LEN + name_len;


>                 } else {
>                         s += slen;
>                 }
> @@ -128,11 +151,47 @@ ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
>         return res;
>  }
>
> +static char *ovl_xattr_escape_name(const char *prefix, const char *name)
> +{
> +       size_t prefix_len = strlen(prefix);
> +       size_t name_len = strlen(name);
> +       size_t escaped_len;
> +       char *escaped, *s;
> +
> +       escaped_len = prefix_len + OVL_XATTR_ESCAPE_PREFIX_LEN + name_len;
> +       if (escaped_len > XATTR_NAME_MAX)
> +               return ERR_PTR(-EOPNOTSUPP);
> +
> +       escaped = kmalloc(escaped_len + 1, GFP_KERNEL);
> +       if (escaped == NULL)
> +               return ERR_PTR(-ENOMEM);
> +
> +       s = escaped;
> +       memcpy(s, prefix, prefix_len);
> +       s += prefix_len;
> +       memcpy(s, OVL_XATTR_ESCAPE_PREFIX, OVL_XATTR_ESCAPE_PREFIX_LEN);
> +       s += OVL_XATTR_ESCAPE_PREFIX_LEN;
> +       memcpy(s, name, name_len + 1);
> +
> +       return escaped;
> +}
> +
>  static int ovl_own_xattr_get(const struct xattr_handler *handler,
>                              struct dentry *dentry, struct inode *inode,
>                              const char *name, void *buffer, size_t size)
>  {
> -       return -EOPNOTSUPP;
> +       char *escaped;
> +       int r;
> +
> +       escaped = ovl_xattr_escape_name(handler->prefix, name);
> +       if (IS_ERR(escaped))
> +               return PTR_ERR(escaped);
> +
> +       r = ovl_xattr_get(dentry, inode, escaped, buffer, size);
> +
> +       kfree(escaped);
> +
> +       return r;
>  }
>
>  static int ovl_own_xattr_set(const struct xattr_handler *handler,
> @@ -141,7 +200,18 @@ static int ovl_own_xattr_set(const struct xattr_handler *handler,
>                              const char *name, const void *value,
>                              size_t size, int flags)
>  {
> -       return -EOPNOTSUPP;
> +       char *escaped;
> +       int r;
> +
> +       escaped = ovl_xattr_escape_name(handler->prefix, name);
> +       if (IS_ERR(escaped))
> +               return PTR_ERR(escaped);
> +
> +       r = ovl_xattr_set(dentry, inode, escaped, value, size, flags);
> +
> +       kfree(escaped);
> +
> +       return r;
>  }
>
>  static int ovl_other_xattr_get(const struct xattr_handler *handler,
> --
> 2.41.0
>




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